From Automotive Handbook 23rd, updated and expanded edition, Braunschweig, Wiesbaden, Viehweg, 1999, ISBN 3-528-03876-4, pp. 116 and 117, micromechanical hot-film air mass meters of the smallest dimensions are known, that work according to thermal principles. In these, heating and measuring resistors are sputtered, i.e. vapor-deposited, as thin Pt layers onto a silicon chip as carrier. The silicon chip is accommodated in the region of the heating resistor on a micromechanically thinned region of the carrier that is similar to a pressure sensor diaphragm, for the thermal decoupling from its mounting support. The heater resistor element is controlled to a constant overtemperature by the closely adjacently accommodated heating temperature sensor as well as by the air temperature sensor. The air temperature sensor is located inside a thickened edge region of the silicon chip. By contrast to other techniques, in the case of a micromechanical hot-film air mass meter, the heating current is not used as the output signal, but rather the temperature difference of the measured medium established by two temperature sensors, in the case of air mass meters of the air. One of the temperature sensors situated in front of the heating resistor element, as seen in the flow direction of the air, and one behind it. In contrast to the heating current, this output variable reflects the flow rate with its correct sign, even if also in a non-linear manner.
Micromechanical hot-film air mass meters, having detection of the flow direction in the case of a pulsating flow, have been designed for recording the load of internal combustion engines having gasoline and Diesel fuel injection. The installation into the air intake system is made, as a rule, between the air filter and a throttle device as a preassembled assembly having a plug sensor and a measuring housing. In the case of hot-film air mass sensors, thermal flow rate meters are involved. The sensor element and its temperature sensors and the heating region are mounted in a flow sheet metal structure. A partial air flow is conducted from the measuring tube past a sensor element through a measuring channel at the plug sensor housing. A two-part flow mesh made of plastic and stainless steel is provided downstream from the measuring channel. An allocation to the entire air mass flowing in the tube takes place via a calibration of the hot-film air mass meter.
It has turned out that the micromechanical hot-film air mass meters used up to now in continuous vehicle operations demonstrated clear characteristics curve drifts both when used in Otto engines and in Diesel engines. This originates from a flow under the sensor chip. If, because of manufacturing tolerances, the joint gap between the air flow sheet metal or mounting sheet metal is too big, a portion of the partial air mass flow flows at the back side of the sheet metal. The gap width of the joint gap is changed by soiling, so that the characteristics curve of the sensor chip changes because of soiling.
An additional reason for the characteristics curve drift is the corrosion of the air flow sheet metal. Corrosion products that form on the air flow sheet metal lead to a changed air flow to the sensor chip, and thus to a drift in its characteristics curve.
An additional cause of the characteristics curve drift is the running out of the sensor gel. The air flow sheet metal is made, as a rule, these days of a folded sheet metal. Because of the folding gap, sensor gel from an adjacent hybrid chamber reaches the vicinity of the sensor chip. By hybrid chamber is understood a circuit substrate on which the entire electronic system is accommodated. This hybrid chamber is filled with silicone gel so as to protect the integrated circuits and the appertaining bonding connections against environmental influences. The sensor gel acts a adhesive agent for dirt. Sensor gel that has exited from the hybrid chamber and dirt particles depositing on it lead to a change in the temperature housekeeping in the diaphragm of the sensor, and consequently lead to a drift of the sensor's characteristics curve.